I have a similar problem as i use the DFX and a Pulse machine both can be used with large coils ie 18'' dia,i have and use the Docs Swingy thingy and also the Minelab GPX variation of the Pro Swing,both do a reasonable job,was able to buy the GPX harness brand new of Ebay but at a greatly reduce price but would not buy the massive price that the Proswing goes for.
What i do is use the Bungee from the Docs harness and use the Minelab Body harness and it works very well,i can swing a full heavy setup for a very long time,i also have the type of harness that Garrett use on the ATX,its a lighter setup type of harness that i use with smaller detector/coil setups anhd once again that works well,but i dont like using the Garrett one with the really heavy hitters as i find it can cause shoulder strain when used for long time durations.
The main thing when using a harness is the mounting point so that you find the perfect point of balance and not have the unit either nose or back heavy,its worth spending a little time experimenting getting the balance right as this will make it so much better when using a harness on a heavy detector.
The price of the Pro-swing harness is the killer of this unit,as if they lowered the price by a small amount they would sell so many more i feel,i can only give some input on how i use a harness and it may help others on choosing which is the best option for them.Up till early last year my detecting time duaration was getting shorter and shorter due to coming of a motor bike in the 70s,but by using a harness i can finally get a full days digging time back again.